Projection displays based on liquid-crystal displays (LCDs) have found wide acceptance throughout many technology fields. Because of their size, cost, and adaptability advantages relative to other types of displays, LCD projection displays are finding greater acceptance in, for example, cockpit avionics systems. When used in flight-critical avionics systems, however, the displays must be designed with a certain level of redundancy in to ensure uninterrupted operation in the event of a failure.
One portion of an LCD projection display that is particularly susceptible to recurring failure is the light source. An arc lamp is commonly used as a light source for an LCD projection display because of ready availability, low cost, and chromaticity requirements. However, arc lamps are notoriously unreliable and short-lived, and a single lamp, by itself, might not be considered a suitable light source for a high-reliability display in certain critical applications. Dual lamp systems have been developed to provide satisfactory redundancy, but such dual lamp systems usually increase the size of the lighting system and may thereby make the system unsuitable for some applications. Furthermore, known dual lamp systems require additional mechanical parts to change from one lamp to the other lamp in case of failure, and the mechanical parts increase the possibility of system breakdown. Lastly, the designs of known dual lamp systems usually create unacceptable levels of insertion loss. What is needed, therefore, is a failsafe illumination system, suitable for use as a light source for an LCD projection display, with a minimum of moving parts and a minimum of insertion loss.
It is therefore an object of the invention to provide a failsafe illumination system for an LCD projection display.
It is another object of the invention to provide a failsafe illumination system that uses a minimum number of moving parts.
It is a further object of the invention to provide a failsafe illumination system that is as efficient as a standard illumination system.
It is a further object of the invention to provide a failsafe illumination system that uses readily available components.
A feature of the invention is the relative orthogonal polarization of two light sources, with the polarization orientation of one of the light sources being rotated to ensure the light output from the system has a constant polarization orientation.
An advantage of the invention is the provision of a compact illumination system with no mechanical moving parts.